1. Field of the Invention
The present invention relates to channel estimation in a wireless communications system, and, in particular, in a UMTS Long Term Evolution (LTE) system uplink.
2. Description of the Related Art
The Universal Mobile Telecommunications System (UMTS) is a high-speed cellular radio system that provides digital data and voice communications. UMTS has recently evolved from 3G systems to 3.5G systems using High-Speed Downlink Packet Access (HSDPA) and High-Speed Uplink Packet Access (HSUPA), and still continues to evolve. The UMTS Long Term Evolution (LTE) protocol is currently being specified in 3GPP Release 8 to ensure its competitiveness for the next ten years and beyond. LTE, which is also known as Evolved UMTS Terrestrial Radio Access (UTRA) and Evolved UMTS Terrestrial Radio Access Network (UTRAN), provides new physical-layer concepts and protocol architectures for UMTS. See, e.g., Application Note 1MA111, “UMTS Long Term Evolution (LTE) Technology Introduction,” Rohde & Schwarz GmbH & Co. KG, available at http://www2.rohde-schwarz.com/en/service_and_support/Downloads/Application_Notes/, hereby incorporated by reference in its entirety.
According to the 3GPP Release 8 standard, the LTE downlink uses Orthogonal Frequency-Division Multiple Access (OFDMA). The LTE uplink uses Single-Carrier FDMA (SC-FDMA), which allows a low-complexity receiver implementation in the base station.
FIG. 1 depicts a simplified block diagram of an LTE uplink 100, including an LTE uplink transmitter 110 (e.g., located in a UMTS mobile terminal) and an LTE uplink receiver 130 (e.g., located in a UMTS base station). In transmitter 110, serial data 112 to be transmitted is quadrature-amplitude-modulation (QAM) modulated by modulator 114 and converted into N parallel streams by serial-to-parallel converter 116. The N parallel streams are then input to an inverse fast Fourier transform (IFFT) unit 118, which uses the N parallel streams as bins to create a block of N OFDM symbols, each mapped to a different subcarrier frequency. The N OFDM symbols output from IFFT unit 118 are serialized by parallel-to-serial converter 120 to create a time-domain OFDM signal, and cyclic extension unit 122 adds to the time-domain OFDM signal a cyclic prefix that provides guard time to reduce intersymbol interference. Finally, the time-domain OFDM signal is transmitted over the LTE uplink.
In receiver 130, the transmitted signal is received and input to cyclic extension unit 132, which removes the cyclic extension. The resulting signal is converted into N parallel OFDM symbols at serial-to-parallel converter 134, and the N parallel OFDM symbols are input to fast Fourier transform (FFT) unit 136, which removes the N subcarrier frequencies and outputs N parallel words. The N parallel words are then input to (i) a parallel-to-serial converter 138, which converts the N parallel words to serial data, and (ii) a channel estimator unit 140, which outputs, for each parallel word, an estimated channel transfer function, based on a locally generated pilot signal and a copy of the pilot signal contained within the received signal. M-point IDFT and equalizer unit 142 then separates the serial data for each channel via an M-point IDFT and equalizes the serial data for each channel, based on the corresponding estimated channel transfer function from channel estimator unit 140. In a single-input, single-output (SISO) system having a single transmission channel from a single transmitter antenna to a single receiver antenna, channel estimator 140 generates an estimated channel transfer function corresponding to the single transmission channel. The equalized serial data for each channel is then demodulated by QAM demodulator unit 144 to recover data 146.
To assist with channel estimation, the LTE uplink protocol includes the transmission of known pilot signals at regular intervals, along with data signals. As described above, channel estimator 140 in receiver 130 uses these transmitted pilot signals to estimate channel characteristics in the LTE uplink. M-point IDFT and equalizer unit 142 in receiver 130 then uses the channel estimates to improve the accuracy of the data reception and demodulation. Conventional techniques for channel estimation, e.g., the Minimum Mean Square Error (MMSE) and Least Squares (LS) techniques, are computationally expensive, however.
An LTE uplink may also include advanced antenna technologies, such as Multiple Input Multiple Output (MIMO). See, e.g., A. Toskala et al., “Utran Long Term Evolution in 3GPP,” IEEE 17th International Symposium on Personal, Indoor and Mobile Radio Communications, pp. 1-5, September 2006, hereby incorporated by reference in its entirety. In a MIMO-based system, there are at least two transmitter antennas and at least two receiver antennas.
FIG. 2 represents a two-by-two MIMO LTE system 200 having two transmitter antennas 202 and 204, which transmit signals over four transmission paths 206, 208, 210, and 212 to two receiver antennas 214 and 216, where each receiver antenna 214, 216 receives a signal corresponding to the superposition of signals arriving over two different transmission paths. For example, the signal received at receiver antenna 214 corresponds to the superposition of (i) the signal transmitted from transmitter antenna 202 via transmission path 206 and (ii) the signal transmitted from transmitter antenna 204 via transmission path 208. In general, each transmitter antenna 202, 204 is associated with its own transmitter analogous to transmitter 110 of FIG. 1, and each receiver antenna 214, 216 is associated with its own receiver analogous to receiver 130 of FIG. 1.
In order to separate the transmitted pilot signals from each MIMO transmitter antenna 202, 204 received at each receiver antenna 214, 216, MIMO LTE system 200 may employ cyclic-shift transmit diversity (CSTD). CSTD is an adaptation of the idea of delay diversity to OFDM systems. With CSTD, each antenna element in a transmit array sends a circularly shifted version of the same OFDM time-domain symbol. For each transmitter antenna, a cyclic prefix is added after circularly shifting the OFDM symbol. See, e.g., Javvin Technologies, Inc., Wireless Technology Terms, Glossary and Dictionary, at http://www.javvin.com/wireless/CSTD.html. For example, constant-amplitude zero-autocorrelation (CAZAC) sequences may be employed to provide a CSTD transmission scheme. Thus, in MIMO LTE system 200, the signal transmitted by transmitter antenna 204 is a circularly shifted version of the signal transmitted by transmitter antenna 202.
The pilot signals received at each receiver antenna may be separated by converting the OFDM signals into the time domain and performing windowing operations based on the cyclic shift. For example, U.S. Patent Application Publication No. US 2008/0031375 filed by Zhou et al. (hereinafter, “Zhou”), hereby incorporated by reference in its entirety, describes a channel estimation and separation method in a MIMO-OFDM system. The Zhou method includes (a) Fourier-transforming a plurality of signals received by a receiving antenna; (b) performing channel estimation by a least-squares method based on a known pilot signal; (c) inverse-Fourier-transforming the least-squares channel estimates into an impulse response in the time domain; (d) separating the transformed impulse response into channel impulse responses of the respective signals by use of a window function having a low-pass filter characteristic; and finally (e) obtaining frequency-domain estimated transfer functions for each channel by Fourier-transforming each of the channel impulse responses. The Zhou method, however, is computationally expensive and requires extensive processing resources.